While our ocean may seem like one giant body of water, its temperatures fluctuate widely. Temperatures can range from below freezing in the deep waters of Antarctica to more than 100°F in shallow coastal waters and everywhere in between. And unlike humans, marine animals can’t just blast the air conditioning or put on a fleecy jacket when they get uncomfortable. Rather, years of evolution have equipped these creatures with several remarkable ways to regulate body temperature and survive in extreme conditions.
Thermoregulation—the process by which animals (and even humans!) maintain their internal body temperatures—is critically important to their survival. Discover how these amazing animals have adapted to life in extreme temperatures.
How do leatherback sea turtles stay warm in cold seas?
Sea turtles are cold-blooded (also known as ectothermic), which means that they rely on external sources of heat. This is why you’ll see turtles, alligators and other reptiles basking in the sun to warm up or certain fish burrowing in the bottom of a lake during freezing months. So, how are leatherback sea turtles able to survive when following swarms of sea jellies to deep, near-freezing waters as far north as Norway?
Leatherbacks rely on an adaptation strategy called countercurrent heat exchange that helps them conserve body heat. The arteries responsible for carrying warm blood from the heart sit right beside the veins bringing in cooler blood from the limbs—the warmth from the arteries transfers to that cooler blood to warm it up before it flows back throughout the body. This exchange can also be reversed when leatherbacks swim back south during nesting season.
In addition to the countercurrent heat exchange, leatherbacks are also gigantothermic. This means that leatherbacks, the largest sea turtle species, can maintain a steady, warm body temperature. Their large body size combined with thick layers of oily, insulating fat and a leathery shell traps internal heat and reduces heat loss.
Are bluefin tuna cold-blooded?
Speaking of heat exchange… While most fish are ectothermic, there are a few that are regionally warm-blooded, or endothermic. Bluefin tuna, for example, are able to conserve heat in specific parts of their body (muscles, viscera, eyes and brain) while their heart remains cold, but not without significant energy expenditure and they cannot keep this up consistently. The bluefin tuna’s heat exchange enables a dense, intertwined system of blood vessels, also known as a “rete mirabile” or “wonderful net” that can reclaim heat. This allows them to hunt in colder waters and maintain their fast reflexes that help them preserve their top predator status.
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Why don’t fish freeze in Antarctica?
Similar to how antifreeze works in your car by disrupting water’s ability to form ice crystals, preventing your car engine from cracking under sub-zero temperatures, certain species of fish create specialized “antifreeze” proteins that prevent the formation of ice in the fish cells and body fluids. Rather than resisting these extreme temperatures that would otherwise freeze them in place, certain fish have developed a rather marvelous adaptation using Antifreeze Glycoproteins (AFGPs). These AFGPs circulate in the bloodstream, disrupting the water molecules to the point where they cannot bond together to form crystals. AFGPs are found in Antarctic notothenioids like the Antarctic toothfish, dragonfish and icefish, as well as the northern cod.
Additionally, Antarctic icefish are the only vertebrates known to have no red blood cells (RBCs) Yep—you read that right. These fish were discovered in 1928 with milky-white blood containing precisely zero hemoglobin. So, how do these fish get the oxygen required for all living creatures on Earth? Antarctic icefish absorb oxygen directly into their blood plasma by diffusing it through their gills and super thin skin. Their wide blood vessels and large hearts allow their very cold and watery blood to move quickly and easily through their circulatory system. No RBCs and antifreeze proteins? Antarctic icefish are an evolutionary marvel. And it appears they’re not alone. In 2025, scientists discovered Asian noodlefishes have completely lost their myoglobin and hemoglobin genes.
What ocean animals can survive on the floor of the deep sea?
Not many animals have what it takes to survive on the sea floor. However, there are a few creative communities of critters that manage to survive by taking advantage of the special circumstances on the sea floor. Enter hydrothermal vents. Hydrothermal vents are like deep-sea hot springs that form in areas with underwater volcanic activity, where moving tectonic plates create fissures in the ocean floor. Magma-heated water escapes from inside the earth through these cracks in the seafloor, releasing a buffet of rich minerals from Earth’s crust, like sulfur and calcium.
What creature could thrive in waters that fluctuate from near freezing to more than 400°F when these vents erupt? Riftia tubeworms, of course! Commonly known as giant tubeworms or Riftia pachyptila, these worms strategically place themselves in mixed-temperature zones, mere inches between hydrothermal vents. These worms can grow to eight feet long and have a symbiotic relationship with chemosynthetic bacteria. The tube worm absorbs oxygen and hydrogen sulfide through its bright red appendage called a plume, which the bacteria then use in the process of chemosynthesis. In turn, the worm receives energy from the bacteria.
Other animals live on the vents, too, including other worms, deep-sea mussels, gastropods and deep-sea octopuses. There are even zoarcid fish that specialize in preying on invertebrates that live on hydrothermal vents. Unlike other deep-sea creatures that need to withstand very cold waters, these guys need to be able to handle the heat—the water around hydrothermal vents can reach up to 660 degrees Fahrenheit!
As you can see, thermoregulation can be the difference between life and death in extreme environments throughout the ocean. While these adaptations showcase incredible evolutionary wonders, they also help scientists understand how species respond to a changing climate. Threats like historic marine heatwaves, intense hurricanes fueled by warming waters and mass coral bleaching events paint a clear picture: The climate crisis is here, and it will only get worse without immediate action. Join us in calling for our elected leaders to take action to mitigate climate change now.
